1. Field of the Invention
The present invention relates to low noise converters, particularly a low noise converter employed as an LNB (Low Noise Block down-converter) incorporated in a receiver of an antenna for satellite signal transmission/reception, converting two input signals differing in polarization into signals of different intermediate frequencies (IF) for transmission to an indoor unit (indoor receiver), and a receiver apparatus including such a low noise converter.
2. Description of the Background Art
FIG. 14 is a block diagram showing a first conventional LNB. The LNB of FIG. 14 is directed to satellite broadcasting/communication reception that allows reception of a plurality of polarized waves, employed for receiving the Ku band satellite signals of the United States.
According to FIG. 14, a reception signal of input frequency 11.7 GHz–12.2 GHz is divided into a horizontal polarization signal and a vertical polarization signal by a cross polarization separator (not shown), which are applied to input terminals 1 and 2, respectively. The signals applied through these two input terminals are respectively picked up by an antenna probe. Each input signal is amplified with low noise by low noise amplifiers LNA 3–6, and then passes through BPFs (Band Pass Filter) 7 and 8 functioning to pass through signals of a desired frequency band and removing signals of an image frequency band.
Then, one signal is applied to a mixer circuit (mixer) 11 together with an oscillation signal of 10.75 GHz output from a local oscillator 209. At mixer 11, the signal is frequency-converted into a signal of an intermediate frequency band-of 950 MHz–1450 MHz. The other signal is applied to a mixer 12 together with an oscillation signal of 10.15 GHz output from a local oscillator 210. At mixer 12, the signal is frequency-converted into a signal of the intermediate frequency band of 1550 MHz–2050 MHz. The signals of these two bands are combined at a combine circuit 215 and transmitted to an IF amplifier 16 to be amplified so as to have appropriate noise and gain characteristics for output from one IF output terminal 17.
IF output terminal 17 receives via a coaxial cable a direct current voltage superimposed on an output signal from an indoor receiver (not shown). A power supply circuit 18 converts that direct current voltage into a predetermined potential, which is applied to respective circuits.
FIG. 15 is a block diagram of a second conventional LNB. The LNB of FIG. 15 receives at input terminals 1 and 2 two signals differing in polarization as signals from a satellite having the frequency of 12.2–12.7 GHz. When the signals from a satellite correspond to a right-hand polarized wave and a left-hand polarized wave, the signals pass through a transducer connected at a preceding stage to the converter to convert the two circularly-polarized waves into two linearly-polarized waves, which are applied to the two input terminals of the converter.
The subsequent operation is similar to that of the first conventional LNB. The signals pass through low noise amplifiers 3–6 and BPFs 7 and 8 which are the band filters for image suppression. Then, one is applied to mixer 11 to which a local oscillation signal having the oscillating frequency of 11.25 GHz is applied from a local oscillator 209A. The other is transmitted to mixer 12 to which a local oscillation signal having an oscillating frequency of 14.35 GHz is applied from a local oscillator 210A. The signals are further transmitted to a LPF (Low Pass Filter) 13 having a transmitting frequency of 950–1450 MHz and to an HPF (High Pass Filter) 14 having a transmitting frequency of 1650–2150 MHz, respectively. The signals of these bands are combined at combine circuit 215, and then applied to IF amplifier 16 to be amplified and output from IF output terminal 17, likewise the first conventional LNB.
In the LNB of FIG. 14, harmonic components of 1200 MHz and 1800 MHz that are two times and three times, respectively, the frequency difference of the local oscillation signals output from the two local oscillators 209 and 210 (10.75 GHz–10.15 GHz=0.6 GHz) are included as spurious harmonic components in the IF bands of 950 MHz–1450 MHz and 1550 MHz −2050 MHz, respectively. Some measures must be taken to suppress this level.
Possible measures include enhancing the shields of the two local oscillators 209 and 210 as well as the two mixers 11 and 12. However, this approach will render the structure complex and increase the cost. It is to be noted that the two IF bands converted at the two different local oscillators 209 and 210 are in the ranges of 950 MHz–1450 MHz and 1550 MHz–2050 MHz, respectively, and the guard band frequency band therebetween is 1450 MHz–1550 MHz. In other words, this guard band is 100 MHz, which is narrow. In order to take advantage of this guard band frequency band at LPF 13 and HPF 14 prior to obtaining signals of respective IF bands to suppress the noise level of one IF band from affecting the other frequency band, an LPF and an HPF superior in cut-off characteristics will be required. Such requirements will increase the cost.
In the second conventional LNB of FIG. 15, the frequency difference of the local oscillation signals output from local oscillators 209A and 210A is 3.1 GHz, and a fourfold harmonic component thereof is 12.4 GHz, This becomes a spurious harmonic component for a signal of 12.2–12.7 GHz received from a satellite, acting as an interference wave with respect to a reception signal of 12.4 GHz. To suppress such a wave, the two local oscillators 209A and 210A or mixers 11 and 12 must be subjected to some measures such as shield enhancement, likewise the first conventional LNB. Thus, the cost will be increased.